"write a cgi (can be perl, asp etc.) on the server that makes the call (I’m guessing you’ll have more than one location to call) get the data and respond to the client’s browser. In that mode you can implement a proprietary protocol between cgi and 8051"

That means you not only have to develop, test, deploy, support, maintain, etc the embedded stuff, but also the CGI, the server, the proprietary protocol, ...
Might mean you have to learn CGI and server admin; maybe extra hosting costs, ...

So when you consider the Total System it may not be quite such a small nut as you first thought!

But you are quite right to say, "think about it" - we should always consider all the options, constraints, etc to seek the Best solution.

"PS I was founding somewhere a nullmodem.inf driver"

You just need to set-up your Dialup Networking to use "Dial-Up Networking Serial Cable between 2 PCs" (that's what it's called on Win-NT4, anyway).